The use of the primary and secondary synchronization channels in the 3GPP WCDMA system allows the user equipment to achieve initial synchronization to the serving cell. Once past this step the synchronization channels are no longer needed, but their presence still generates interference to the other channels.
In the 3GPP wideband CDMA (WCDMA) system, Orthogonal Variable Spreading (OVSF) codes are used to allow multiplexing of different channels and multiple access for sharing of resources among users. Because of their orthogonality, the use of OVSF codes means no inter-code interference is created.
An exception to this however is the Primary and Secondary Synchronization Channels (PSCH and SSCH) which are not scrambled with the cell specific scrambling code (section 5.3.3.5, 3GPP TS 25.211, “Technical Specification Group Radio Access Network; Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, December 2005). All the channels other than the synchronization channels are allocated OVSF codes and then scrambled. The scrambling sequence is a pseudo random sequence designed to randomize the inter-cell interference. Because the OVSF codes are scrambled with the same scrambling code they stay orthogonal. The synchronization channels on the other hand are not scrambled (and also allocated different codes) and hence are not orthogonal. This means the synchronization channels can cause interference with the rest of the channels.